I'm afraid MVC might lead to a
complex\lengthy system where the flow
is lengthy. Will I get lost? Is it
scalable if I organize well?
Well, part of that depends on how quickly you pick up the technology.
Overall, ASP.NET MVC is just as scalable as any WebForms solution, and, in my limited experience, probably even more so. You have to remember that MVC is a pattern that is not new - it has been utilized by application developers for years. It is not even really ALL that new to web development, although ASP.NET MVC makes it very easy to do which is one of its appeals.
Some of the traditional advantages of ASP.NET MVC, such as testability for example, go a long way to making the overall project easier to maintain and quicker to build. I built my first ASP.NET MVC website within a week's time, hooking into libraries that had been developed years before, but providing a front-end web-interface. It really was that simple.
The question you asked of: "Will I get lost?" really depends on you and the application you are developing, but only you can answer that. You do have to be willing to put time into learning and understanding MVC...because you are coming from a WebForms background, there will be changes that won't be familiar to you.
Either way, I personally recommend the use of MVC, but, as always, it depends on your project needs.
Update: My own experience with MVC has been extremely positive. I know and understand WebForms just because of using it for the past few years, but I recently started a project at my company and, because I was given the leeway to choose the technologies that I wished to use, I decided it would be interesting to try the MVC model within a web application (the web application is a requirement). My team and I had experience with MVC in desktop applications, but we were all inexperienced with web development in general (as a whole).
The ASP.NET MVC experience was extremely smooth and it was an easy transition for my team to make. I ordered some books for the team, we spent a bit of time coming up to speed in our understanding, and we then began to develop (requirements had already been completed before this step - we were at implementation phase). Like I said previously, it took a week and we had already made great leaps forward - productivity was, and still is, very high. I am very pleased with the MVC model.
You appear to be concerned that the MVC model is going to cause your application to turn into a pile of mush, or to become unwieldy to maintain. As a whole, this is far from the truth. The entire idea of separated concerns makes MVC ideal for scalability of an application, as well as increasing its maintainability and testability. With that said, a lot of how well the application grows depends on your team's understanding of the MVC paradigm and their skills as developers. MVC is not for everybody, nor is it for every project. There are also many other factors to take into consideration with respect to scalability that has nothing to do specifically with MVC (design of the database for example has a HUGE impact - probably more than whatever development paradigm you choose). I personally like ASP.NET MVC, and have had a very good experience with it, but you have to take into consideration all the aspects of your project (team members, time allotted, budget, etc) to make a final decision.